(1) Field of the Invention
The present invention relates generally to the treatment of a web to obtain desired physical characteristics and particularly to a method and apparatus for controlling multiple steam applicators used to distribute steam against the web.
(2) Description of the Related Art
One of the parameters used in grading a web is the gloss of the surface. During paper production, various grades of paper having different surface gloss are produced to suit various applications. Typically, bulk paper is produced in a continuous sheet and wound in rolls having dimensions of 12 to 36 feet in the cross-direction (i.e., across the width of the sheet). Uniformity of gloss on the paper surface is often desirable or necessary. For example, where the roll of paper is cut to size for making various products, the consistency of the gloss of the individual products is dependent upon the uniformity of the gloss of the original bulk paper roll.
Paper production typically involves a calendering process which includes pressing paper between calender rolls to obtain desired physical characteristics. Calendering paper can change its density, thickness, and surface characteristics, including gloss. Steam is frequently applied to paper being calendared so as to moisten and heat the paper and thereby affect certain characteristics. For example, gloss is typically enhanced on the surface of paper by applying steam to the paper surface, followed by pressing the paper between a series of calender rolls, arranged in a stack of alternating hard polished steel rolls and soft or resilient rolls made of cotton. The paper absorbs the steam, and paper fibers at the surface are softened by the heat and moisture. As the steam treated paper surface comes into contact with the calender rolls, it is smoothed by the pressing and rubbing actions of the polished steel roll and the adjacent cooperating soft roll. The degree of gloss enhancement is dependent on the amount of steam applied to the surface.
A common problem encountered in making a glossy finish using steam treatment is its non-uniformity. Localized variations in the amount of steam applied to the surface of the paper may affect the uniformity of the gloss finish. Also, there are other variables in the calendering process such as temperature and calender roll pressure which may affect the amount of steam required for a particular degree of gloss. A more uniform gloss finish can be obtained if the amount of steam directed at different cross-directional sections of the paper surface can be controlled.
Another common problem associated with the application of steam in creating a gloss finish is that excess steam which has not been absorbed by the paper condenses on cool surfaces of the adjacent structure of the calender stack. For example, the steam may condense on a steel calender roll, which will wet the paper as the steel roll contacts the paper. The extra moisture from the steel calender roll in addition to the moisture applied directly to the sheet from the steam applicators will affect the moisture distribution and hence the gloss finish and other physical properties of the paper. In addition, excess steam may condense on a cool portion of the paper surface where steam treatment is not intended, thereby affecting the gloss profile.
Moreover, steam which condenses on these cool surfaces forms water droplets which may drip on the paper as it passes through the calender stack, thereby affecting the desired properties of the paper. Also, since gloss formation is generally the final step in paper manufacturing, the application of excess steam can cause coating detachment of any previously applied water soluble coatings which then adhere to the calender roll.
In the past, various apparatuses have been used for steam applicators to distribute steam on paper sheets during calendering. An apparatus designed for applying steam to achieve cross-directional gloss control is disclosed in U.S. patent application Ser. No. 07/303,494 of Boissevain, et al., filed Jan. 27, 1989, now U.S. Pat. No. 4,964,311 entitled, Cross-Directional Steam Application Apparatus. The disclosed apparatus functions by discharging jets of steam through a plurality of bucket nozzles located along the length of a manifold pipe.
Gloss formation depends upon the amount of steam that can be absorbed by the paper. One technique for increasing the total amount of steam that can be absorbed is to apply the steam at several different locations during the calendering process. The present invention provides for the use of multiple steam applicators spaced at intervals from one another along the direction of sheet movement and placed adjacent to the calender stack. Although the use of multiple steam applicators permits a greater degree of gloss enhancement, there is a need for a controller for the multiple steam applicators.